Update static file routing and root endpoint for frontend integration

Update Python subprocess termination to use SIGKILL instead of SIGTERM for forceful termination

Replace `ffmpeg`-based audio loading with `soundfile` and `librosa`

Add macOS entitlements and assets for Electron app build

Add Electron app scaffolding with loading screen and Python backend integration

Remove unused assets: delete `appicon.png` and `dmg_bg.png` from `electron-app`.

Organize assets: rename icons to `electron-app/assets/icons` and add `dmg_bg.png` to `electron-app/assets/images`.